Tappets in an internal combustion engine function to transmit the rotational movement of the engine camshaft into the linear movement needed to move the pushrods and rocker levers which actuate fuel injectors and intake and exhaust valves. Many different types of tappets are known which perform this function. One type of tappet widely used in internal combustion engines today, which may be called a roller-type tappet, includes a roller mechanism at one end for reducing frictional losses between the tappet and the associated cam surface on the camshaft. It has long been recognized that proper alignment of the tappets with respect to the camshaft and pushrods and maintenance of this alignment is essential to the efficient operation of the engine. However, there is a tendency for roller type tappets to rotate during engine operation and become misaligned, which may result in cam and tappet failures. Replacement or repair of these engine components is an extremely lengthy procedure since much of the engine must be disassembled to allow access to the tappets and cams. Moreover, such a process is very costly to the manufacturer if the engine is still under warranty or to the engine owner if the warranty is no longer in effect.
There have been many suggestions made for ways to maintain proper tappet alignment and prevent rotation of roller type tappets during engine operation. While these suggestions superficially appear to be simple solutions to the problem, a closer look reveals that they are in reality complex and accompanied by problems. One early approach was to provide a tappet and corresponding hole in the engine block in a square shape, so that tappet rotation and misalignment was not likely to occur. However, providing a square hole in the engine block and a square tappet to fit in it is both difficult and costly.
A second approach is exemplified by U.S. Pat. Nos. 1,802,330 and 3,089,472, which disclose a housing mounted in the engine block which receives the body of the tappet. U.S. Pat. No. 1,802,330 specifically discloses a mating cylindrical housing with the tappet slidably mounted in the bore of the housing which maintains the tappet in alignment. A pin inserted through slots in a pair of adjacent tappets holds them against rotation. The structure disclosed in U.S. Pat. No. 3,089,472 includes an alignment plate which can be temporarily or permanently fastened to the engine block to achieve the desired alignment of a pair of tappets in their housings. Once this is achieved, the rollers at the ends of the tappets which engage the cam and transmit movement to the pushrods should be properly aligned and unlikely to rotate. Both of the aforementioned structures are fairly effective in preventing tappet rotation. However, the utilization of such tappet housings increases the number of engine parts subject to costly repair or replacement. In addition, the cost associated with the additional machining required to bore these tappet housings makes this solution to the problem of maintaining optimum tappet alignment an expensive one.
More simplified structures, such as the ones disclosed in U.S. Pat. Nos. 2,846,988 and 3,108,580, which eliminate the tappet housing and provide elements to prevent rotation have been proposed to maintain tappet alignment. Openings in the engine block guide the vertical movement of the tappets, thus eliminating the need for a tappet housing, and two adjacent tappets are prevented from rotating by the use of an antirotational device. The device disclosed in U.S. Pat. No. 2,846,988 includes two offset metal stampings and a flat piece which are fitted together and inserted into corresponding slots in two adjacent tappets. However, the clearance between the portions of these three pieces required to assemble the antirotational device and still permit vertical tappet movement may be sufficient to permit some rotation of the tappets and, subsequently, misalignment. Furthermore, the requirement for the three separate pieces which form this device, two of which must be precisely stamped to achieve the desired result, adds to the cost of manufacturing, assembling and replacing such a device. The means for preventing tappet rotation disclosed in U.S. Pat. No. 3,108,580 comprises an alignment bar or plate which is secured to the engine block and includes projections shaped to extend around approximately three quarters of the circumferences of two adjacent tappets, preventing the rotation thereof as they are guided upward through the engine block. Although such a structure adequately serves the desired purpose, the manner in which it is fastened to the engine block requires four separate attaching elements, creating a substantial assembly problem and, as with the other structures already discussed, adding significantly to the cost.
The assignee of the present invention currently utilizes an antirotational tappet guide plate which is secured to the engine block by two capscrews. However, maintaining the tolerance for the tappet guide screw holes within required limits during machining of the engine block is difficult and if not done properly results in the failure of the guide plate to prevent tappet rotation. Moreover, the time required on the assembly line to apply the washers and capscrews which secure the guide plate and then torque these screws is longer than is desirable.